Tolerance to achieve. The algorithm terminates when either the relative
or the absolute residual is below tol.

maxiter : integer

Maximum number of iterations. Iteration will stop after maxiter
steps even if the specified tolerance has not been achieved.

M1 : {sparse matrix, dense matrix, LinearOperator}

Left preconditioner for A.

M2 : {sparse matrix, dense matrix, LinearOperator}

Right preconditioner for A. Used together with the left
preconditioner M1. The matrix M1*A*M2 should have better
conditioned than A alone.

callback : function

User-supplied function to call after each iteration. It is called
as callback(xk), where xk is the current solution vector.

xtype : {‘f’,’d’,’F’,’D’}

This parameter is DEPRECATED – avoid using it.

The type of the result. If None, then it will be determined from
A.dtype.char and b. If A does not have a typecode method then it
will compute A.matvec(x0) to get a typecode. To save the extra
computation when A does not have a typecode attribute use xtype=0
for the same type as b or use xtype=’f’,’d’,’F’,or ‘D’.
This parameter has been superceeded by LinearOperator.